Electrode joint locking system

ABSTRACT

An electrode joint having first and second complementary elements capable of being joined together to form the joint, wherein one of the threaded elements has at least one slot at least partially along its length; and wherein one of the threaded elements includes a source of a flowable adhesive in fluid communication with the slot.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a locking system for electrode joints.More particularly, the invention concerns a unique system forfacilitating the mechanical locking of electrode joints using anadhesive material.

2. Background Art

Graphite electrodes are used in the steel industry to melt the metalsand other ingredients used to form steel in electrothermal furnaces. Theheat needed to melt metals is generated by passing current through oneor a plurality of electrodes, usually three, and forming an arc betweenthe electrodes and the metal. Electrical currents in excess of 100,000amperes are often used. The resulting high temperature melts the metalsand other ingredients. Generally, the electrodes used in steel furnacesare used in electrode columns, that is, a series of individualelectrodes joined to form a single column. In this way, as electrodesare depleted during the thermal process, replacement electrodes can bejoined to the column to maintain the length of the column extending intothe furnace.

Conventionally, electrodes are joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections, with at least one end of each of the electrodes comprisingfemale threaded sections capable of mating with a male threaded sectionof the pin. Thus, when each of the opposing male threaded sections of apin are threaded into female threaded sections in the ends of twoelectrodes, those electrodes become joined into an electrode column.Commonly, the joined ends of the adjoining electrodes, and the pintherebetween, are referred to in the art as a joint.

Alternatively, the electrodes can be formed with a male threadedprotrusion or tang machined into one end and a female threaded socketmachined into the other end, such that the electrodes can be joined bythreading the male tang of one electrode into the female socket of asecond electrode, and thus form an electrode column. The joined ends oftwo adjoining electrodes in such an embodiment is referred to in the artas a male-female joint.

Given the extreme thermal and mechanical stress that the electrode andthe joint (and indeed the electrode column as a whole) undergo,detachment of the joint and subsequent loss of the electrode columnbelow the detached joint is a recurring problem. In so-called non jammedjoints, where the threads of the pin and electrodes, or the twoelectrodes in a male-female joint, meet on only one of the thread faces,solutions have been proposed, where pitch or another material is allowedto melt and then infiltrate between the threads, where it carbonizes inthe heat of the furnace and forms a bond between the joint elements.

For instance, in International application PCT/US02/10125, inventorsPavlisin and Weber disclose a “plug” formed of pitch and expandablegraphite. When the plug is placed at the base of an electrode socket,the heat of the furnace causes the pitch to melt and the graphite toexpand, forcing the melted pitch between the threads where it carbonizesand locks the joint together. Another joint locking system employed inthe past has been to provide one or more holes in an electrode pin at ornear each of its ends, and positioning pitch in the holes. Again, theheat of the furnace causes the pitch to melt and flow across the threadswhere it carbonizes and locks the joint in position.

Although effective, these prior art methods for joint locking are onlymaximally effective in non-jammed threads, such as are illustrated inFIG. 1 a. In fully jammed threads, illustrated in FIG. 1, where bothfaces of the threads of one element contact the threads of the otherelement, there is insufficient space between the threads to permit thepitch or other adherent composition to flow between the threads. Thereexists a need, therefore, for an improved locking mechanism for fullyjammed graphite electrode joints.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a system for lockinga joint for graphite electrodes.

It is another aspect of the present invention to provide a joint forgraphite electrodes which is designed to better withstand the thermaland mechanical stresses on an electrode column in use, as compared toart-conventional graphite electrode joints.

It is yet a further aspect of the present invention to provide a jointfor graphite electrodes which produces electrode column joints havingimproved strength and stability.

Still another aspect of the present invention is a graphite electrodejoint, having improved resistance to stub loss, defined as the loss ofthe part of the electrode column lying from the arc tip (that is, theend or tip of the electrode column extending into the furnace and fromwhich the arc is formed) to and sometimes including the joint closest tothe arc tip, as compared to art-conventional graphite electrode joints.

These aspects and others that will become apparent to the artisan uponreview of the following description can be accomplished by providing anelectrode joint formed from first and second complementary elements,such as graphite electrodes, capable of being joined together to formthe joint, wherein one of the threaded elements has at least one slot(and preferably a plurality of slots) at least partially along itslength; and further including a source of a flowable adhesive in fluidcommunication with the slot. The source of flowable material can furthercomprise a flow-enhancing material. Preferably, the first and secondcomplementary elements comprise a male threaded element, such as a pinor a male tang, and a female threaded element, such as a female socket,which can be threadedly joined to form the joint.

The flowable adhesive advantageously comprises pitch, and is present asa plug disposed at the base of the female threaded element.Alternatively, one of the first and second complementary elements canhave an adhesive-containing shaft formed therein and in fluidcommunication with the slot.

It is to be understood that both the foregoing general description andthe following detailed description provide embodiments of the inventionand are intended to provide an overview or framework of understanding ofthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention and are incorporated in and constitute a part of thespecification. The drawings illustrate various embodiments of theinvention and together with the description serve to describe theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side cross-sectional view of a fully jammedmale-female graphite electrode joint in accordance with the presentinvention.

FIG. 1 a is a partial side cross-sectional view of a non-jammed graphiteelectrode joint.

FIG. 2 is a partial side plan view of a graphite electrode having aslotted male tang for the graphite electrode joint of FIG. 1.

FIG. 3 is a partial side cross-sectional view of a graphite electrodehaving a slotted female socket for the graphite electrode joint of FIG.1.

FIG. 4 is a partial side plan view of a slotted pin for a fully jammedgraphite electrode joint.

FIG. 5 is a perspective side view of an adhesive material plug inaccordance with the present invention.

FIG. 6 is a partial side cross-sectional view of a graphite electrodehaving a slotted male tang with a bore hole, in accordance with thepresent invention.

FIG. 7 is a partial side cross-sectional view of a graphite electrodehaving a female socket with a bore hole, in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Graphite electrodes can be fabricated by first combining a particulatefraction comprising calcined coke, pitch and, optionally, mesophasepitch or PAN-based carbon fibers into a stock blend. More specifically,crushed, sized and milled calcined petroleum coke is mixed with acoal-tar pitch binder to form the blend. The particle size of thecalcined coke is selected according to the end use of the article, andis within the skill in the art. Generally, particles up to about 25millimeters (mm) in average diameter are employed in the blend. Theparticulate fraction preferable includes a small particle size fillercomprising coke powder. Other additives that may be incorporated intothe small particle size filler include iron oxides to inhibit puffing(caused by release of sulfur from its bond with carbon inside the cokeparticles), coke powder and oils or other lubricants to facilitateextrusion of the blend.

Most preferably, the carbon fibers (when used) are preferably present ata level of about 0.5 to about 6 parts by weight of carbon fibers per 100parts by weight of calcined coke, or at about 0.4% to about 5.5% byweight of the total mix components (excluding binder). The preferredfibers have an average diameter of about 6 to about 15 microns, and alength of preferably about 4 mm to about 25 mm, and most preferably lessthan about 32 mm. The carbon fibers used in the inventive process shouldpreferably have a tensile strength of at least about 150,000 psi. Mostadvantageously, the carbon fibers are added to the stock blend asbundles, each bundle containing from about 2000 to about 20,000 fibers.

Preferably, the fibers are added after mixing of the particulatefraction and pitch has already begun. Indeed, in a more preferredembodiment, the fibers are added after at least about half the mix cyclehas been completed, most preferably after at least about three-quartersof the mix cycle has been completed. For instance, if the mixing of theparticulate fraction and pitch takes two hours (i.e., a mix cycle is twohours), the fibers should be added after one hour, or even ninetyminutes, of mixing. Adding the fibers after the mixing has begun willhelp preserve fiber length (which can be reduced during the mixingprocess) and thereby the beneficial effects of the inclusion of fibers,which are believed to be directly related to fiber length.

As noted above, the particulate fraction can include small particle sizefiller (small is used herein as compared to the particle size of thecalcined coke, which generally has a diameter such that a major fractionof it passes through a 25 mm mesh screen but not a 0.25 mm mesh screen,and as compared to the fillers conventionally employed). Morespecifically, the small particle size filler comprises at least about75% coke powder, by which is meant coke having a diameter such that atleast about 70%, and more advantageously up to about 90%, will passthrough a 200 Tyler mesh screen, equivalent to 74 microns.

The small particle size filler can further comprise at least about 0.5%and up to about 25% of other additives like a puffing inhibitor such asiron oxide. Again, the additive should also be employed at a particlesize smaller than that conventionally used. For instance, when ironoxide is included, the average diameter of the iron oxide particlesshould be such that they are smaller than about 10 microns. Anotheradditional additive which can be employed is petroleum coke powder,having an average diameter such that they are smaller than about 10microns, added to fill porosity of the article and thus enable bettercontrol of the amount of pitch binder used. The small particle sizefiller should comprise at least about 30%, and as high as about 50% oreven 65% of the particulate fraction.

After the blend of particulate fraction, pitch binder, etc. is prepared,the body is formed (or shaped) by extrusion though a die or molded inconventional forming molds to form what is referred to as a green stock.The forming, whether through extrusion or molding, is conducted at atemperature close to the softening point of the pitch, usually about100° C. or higher. The die or mold can form the article in substantiallyfinal form and size, although machining of the finished article isusually needed, at the very least to provide structure such as threads.The size of the green stock can vary; for electrodes the diameter canvary between about 220 mm and 750 mm.

After extrusion, the green stock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid pitch coke, to give the article permanency of form, highmechanical strength, good thermal conductivity, and comparatively lowelectrical resistance, and thus form a carbonized stock. The green stockis baked in the relative absence of air to avoid oxidation. Bakingshould be carried out at a rate of about 1° C. to about 5° C. rise perhour to the final temperature. After baking, the carbonized stock may beimpregnated one or more times with coal tar or petroleum pitch, or othertypes of pitches or resins known in the industry, to deposit additionalcoke in any open pores of the stock. Each impregnation is then followedby an additional baking step.

After baking, the carbonized stock is then graphitized. Graphitizationis by heat treatment at a final temperature of between about 2500° C. toabout 3400° C. for a time sufficient to cause the carbon atoms in thecoke and pitch coke binder to transform from a poorly ordered state intothe crystalline structure of graphite. Advantageously, graphitization isperformed by maintaining the carbonized stock at a temperature of atleast about 2700° C., and more advantageously at a temperature ofbetween about 2700° C. and about 3200° C. At these high temperatures,elements other than carbon are volatilized and escape as vapors. Thetime required for maintenance at the graphitization temperature usingthe process of the present invention is no more than about 18 hours,indeed, no more than about 12 hours. Preferably, graphitization is forabout 1.5 to about 8 hours. Once graphitization is completed, thefinished article can be cut to size and then machined or otherwiseformed into its final configuration.

When the electrode joint is one utilizing a pin, the pin is formed in asimilar manner, although the number of pitch impregnation/bake steps maybe higher for a pin in order to provide greater strength. Once formed,the finished article is then machined or otherwise formed into its finalconfiguration for use as a pin.

When a male-female electrode joint is desired, the male tang (and, byextension, the female socket) should advantageously be dimensioned suchthat the tang will provide the required strength in use. Morespecifically, the ratio of the length of the male tang to the diameterof the electrode (referred to as the tang diameter factor) of at leastabout 0.60 is desirably in creating a male-female electrode joint havingimproved stability and commercially acceptable performance. Moreover, aratio of a factor defined by the ratio of the diameter of the male tangat its base to the male tang length (referred to as the tang diameterfactor) should be no greater than 2.5 times the tang factor for anespecially effective joint with a tang factor of about 0.60. Indeed, thetang diameter factor should most preferably vary with the tang factor,such that when a joint with a tang factor higher than 0.60 is produced,the tang diameter factor of the joint should be lower than 2.5 times thestub factor. More specifically, for every 0.01 higher than 0.60 that thetang factor of a joint is, the maximum tang diameter factor should beabout 0.016 lower. Another joint characteristic that can come into playin designing an effective male-female joint is referred to herein as thetaper factor, which is defined as the ratio of the taper (expressed indegrees) of the male tang to the tang factor, which should be at leastabout 15, where the tang factor is 0.85, and should also vary as jointswith different tang factors are produced. For instance, for every 0.01lower than 0.85 that the tang factor of a joint is, the minimum taperfactor should be about 1.25 higher.

As illustrated in FIGS. 1 and 2, the inventive electrode joint comprisesa male-female graphite electrode joint 10, having a male tang 20 at theend of one electrode 10 a and a female socket 30 at the end of theadjoining electrode 10 b,such that the male tang 20 can threadedlyengage the female socket 30 to form joint 10. The engagement of maletang 20 and female socket 30 is in a fully jammed fashion, asillustrated in FIG. 1, such that each face 22 a and 22 b of the threads22 of male tang 20 abuts a face 32 a and 32 b of the threads 32 offemale socket 30.

Male tang 20 is formed such that at least one slot or groove 24 extendsat least partially along its length, as illustrated in FIG. 2. In thepreferred embodiment, a plurality of slots 24 extend at least partiallyalong the length of male tang 20; indeed, in the most preferredembodiment, four slots 24 are arrayed along the length of male tang 20,with each slot disposed about the circumference of male tang 20 atapproximately 90° intervals.

A source of an adhesive material 40 is disposed in joint 10, in alocation contiguous with slot(s) 24. For instance, an adhesive materialplug 42, illustrated in FIG. 5, can be placed at the base of joint 10,as shown in FIG. 1, provided slot(s) 24 extend completely to the end 26of male tang 20, where the end 26 of male tang 20 approaches the base 36of female socket 30. Adhesive material 40 should be such that, under theconditions to which joint 10 is exposed, adhesive material 20 flowsalong slot(s) 24 and forms an adhesive bond with the threads 32 offemale socket 30, to thereby work to prevent unscrewing of joint 10.Advantageously, slot(s) 24 do not extend all the way to the base 27 ofmale tang 20, to avoid flow of adhesive material out of joint 10.

Suitable materials useful as adhesive material employed in source 40include cements and resins having melting temperatures below thetemperature to which joint 10 is exposed in the furnace, but higher thanthe typical storage temperature of electrodes 10 a and 10 b (to preventpremature melting). The suitable cements or resins should be those whichcure or coke at the furnace temperatures, such that, after melting andflowing about threads 32, the material cures or cokes to form thedesired bond. Most preferably, the material comprises pitch, which has amelting temperature below the temperature to which joint 10 is exposedin the furnace, but higher than the typical storage temperature ofelectrodes 10 a and 10 b; pitch also cokes at the furnace temperatures,such that, after melting and flowing about threads 32, it cokes to bondelectrodes 10 a and 10 b of joint 10 together.

Source 40 can comprise other elements besides the adhesive materialitself For instance, a foaming agent such as oxalic acid, along withmetallurgical pitch and carbon fine particles, can be included with theadhesive material, to facilitate flow of the adhesive material alongslot(s) 24. Other materials, such as binders, etc. may also be included,if desired.

Although the source of adhesive material 40 can be provided in the formof a plug 42 disposed at the base of joint 10, other locations for thesource of adhesive material 40 can also be contemplated. For instance,as shown in FIG. 6, one or more bore holes 28 can be formed in male tang20, such that the entrance to each bore hole 28 lies within slot(s) 24;bore hole(s) 28 can have the source of adhesive material 40 therewithin,such that in the furnace the adhesive material flows out of bore hole(s)28 and along slot(s) 24. Similarly, bore hole(s) 38 can be formed infemale socket 30, as shown in FIG. 7, provided the entrance to borehole(s) 38 open into slot(s) 24 formed in male tang 20.

In the same manner that slot(s) 24 can be formed in male tang 20, in amale-female graphite electrode joint, slot(s) 54 can be formed in one orboth of the male ends 50 a and 50 b of a pin 50, illustrated in FIG. 4,for a graphite electrode joint utilizing pin 50 for joinder, with eithera plug 42 or adhesive material-filled shaft or bore hole(s) in eitherpin 50 or female socket 30 employed in the manner described above. Also,as illustrated in FIG. 3, slot(s) 34 can be formed in female socket 30,in either a male-female graphite electrode joint or a pin joint, witheither an adhesive material plug 42 or adhesive material-filled borehole(s) provided in either female socket 30 or male tang 20 (or pin 50,in a pin joint), employed in the manner described above.

Thus, by use of the joint locking system of the present invention, ameans of preventing unscrewing of an electrode joint is provided, evenin fully jammed joints where prior art systems were ineffective.

The disclosures of all cited patents and publications referred to inthis application are incorporated herein by reference.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

1-19. (canceled)
 20. A graphite electrode comprising a graphite bodyhaving a threaded male tang formed in one end of the body, the threadedmale tang comprising at least one slot which extends partially along thelength of the male tang with a slot length less than a length of thethreaded portion of the male tang, the electrode further comprises athreaded socket formed in a second end of the body.
 21. The electrode ofclaim 20 wherein the socket further comprises a source of flowableadhesive.
 22. The electrode of claim 20 wherein the male tang comprisesa tang factor of at least about 0.6.
 23. The electrode of claim 22wherein the ratio of the diameter of the male tang at the base of thetang to the tang length comprises no greater than 2.5 times the tangfactor.
 24. The electrode of claim 22 which comprises a taper factor ofat least about 15 when the tang factor is about 0.85 or less.
 25. Theelectrode of claim 24 wherein for each 0.01 the tang factor is less than0.85, the taper factor increases by about 1.2 greater than
 15. 26. Theelectrode of claim 20 wherein the socket is capable of receiving a tangwith a tang factor of at least about 0.6.
 27. A graphite electrodecomprising a graphite body having a threaded socket formed in one end ofthe body, and at least one slot which extends partially along the lengthof the threaded socket with a slot length less than a length of thethreaded portion of the socket.
 28. The graphite electrode of claim 27further comprising a threaded male tang located at a second end of thebody.
 29. The graphite electrode of claim 27 wherein the tang furthercomprises at least one slot which extends partially along its lengthwith a slot length less than a length of the threaded tang.
 30. Thegraphite electrode of claim 27 further comprising a source of a flowableadhesive located in a base of the socket.
 31. The graphite electrode ofclaim 27 wherein the socket is capable of receiving a tang with a tangfactor of at least about 0.6.
 32. The graphite electrode of claim 28wherein the tang comprises one or more bore holes.
 33. An electrodecolumn wherein at least one of the electrodes in the column comprises agraphite body having a threaded male tang formed in one end of the body,the threaded male tang comprising at least one slot which extendspartially along its length with a slot length less than a length of thethreaded portion of the male tang, wherein the male tang orientedvertically upward.
 34. The column of claim 33 wherein a second electrodein the column adjacent the electrode includes a threaded socketjuxtaposed to the tang of the electrode, the threaded socket of thesecond electrode includes a source of a flowable adhesive material. 35.The column of claim 33 wherein a second electrode in the column adjacentthe electrode includes a threaded socket juxtaposed to the tang of theelectrode and an adhesive located along the slot of the tang and thethreads of the socket.
 36. The column of claim 33 wherein a tang factorof the tang comprises at least about 0.6.
 37. The column of claim 36wherein the ratio of the diameter of the male tang at the base of thetang to the tang length comprises no greater than 2.5 times the tangfactor.
 38. The column of claim 36 which comprises a taper factor of atleast about 15 when the tang factor is about 0.85 or less.